Use utf8_general_ci collation as a default collation

Now, I see mariadb are using utf8_general_ci as a default collation.
- https://mariadb.com/kb/en/mariadb/supported-character-sets-and-collations/

This mean all of Devstack database will be created with utf8_general_ci collation,
so may be, one service/project can be deployed successfully via Devstack
but will be fail with Kolla deployment.

Therefore, we should use above default collation for Kolla-ansible.

Change-Id: Icbb6c15f536fc6986816c58f4fd68bfb95813e46
Closes-Bug: 1680783

ansible/roles/mariadb/templates/galera.cnf.j2

@@ -21,7 +21,7 @@ binlog_format=ROW
 default-storage-engine=innodb
 innodb_autoinc_lock_mode=2
 
-collation-server = utf8_unicode_ci
+collation-server = utf8_general_ci
 init-connect='SET NAMES utf8'
 character-set-server = utf8